The invention relates generally to battery monitoring systems, and in particular, to systems for monitoring the charge state of rechargeable batteries, and especially rechargeable nickel-cadmium (Nicad) batteries.
The voltage appearing across the terminals of a Nicad battery varies only slightly as the battery discharges from its maximum charge capacity to roughly 10% of that capacity. Because of this and because a Nicad battery may be recharged with relative ease, the Nicad battery has become the preferred power source for portable electronic devices utilizing both digital and analog technology. The almost exclusive use of Nicad batteries as power sources by the portable computer and cellular telephone industries illustrates this preference.
However, despite this broad preference, the utilization of Nicad batteries by the portable electronics industry raises a significant concern. Because the voltage appearing across the terminals of a Nicad battery varies only slightly until just before the battery reaches full discharge, the battery's remaining charge capacity cannot be measured accurately by traditional methods (i.e. by monitoring the voltage between the battery terminals). In fact, the voltage between the battery terminals may only be used to provide the user of a device employing the battery minimal notice of an imminent power failure resulting from full battery discharge. For this reason, portable computers and cellular telephones generally include a low battery indicator which notifies a user when the voltage across the battery terminals reaches a prescribed level. This feature allows the user to store any important information or terminate a call prior to a loss of power. However, the feature provides the user little assurance initially that the device will work for any significant amount of time.
Further, the problems resulting from the inability to estimate a Nicad battery's remaining life or charge capacity by traditional methods are augmented by a second characteristic which these batteries exhibit. When Nicad batteries are repeatedly partially discharged and then recharged, the batteries exhibit a "memory effect." In essence, the full charge capacity of the batteries deteriorates to the capacity of actual use. Thus, if the user of a device powered by a Nicad battery repeatedly fails to fully discharge the battery prior to recharging it, the useful capacity of the battery will deteriorate to the extent of actual use. This effect leaves the user in a precarious position. If the user fails to recharge the battery prior to its full discharge, the user runs the risk of having insufficient power the next time the device is used. If, on the other hand, the user charges the battery prior to its full discharge, the user runs the risk of decreasing the useful life of the battery.
It follows that the portable electronics industry would find a device for monitoring and providing an accurate indication of the charge state of a Nicad battery to be quite desireable.